Multi-band tuned antenna



\ 7,1959 I i G. A. BIRD 2 881,430

MULTI-BAND TUNED ANTENNA Filed se t. 20. 1956 4 ZShets-Sheetl omo v 70'INVENTOR Gonna/v 194 F'R'fl 80w QYKWMW ATTORNEYS April 7, 1959 G. A.BIRD MULTI-BAND TUNED ANTENNA Filed Sept. 20, 1956 2 Sheets-Sheet 2 w a2 v o 5 3 3 m k ,W.\o O Z 9 a 2 7 6 7 3 3 3 v 8 a z\ INVENTQR GORDONHAP/P50 BIRD BYM W AT'rQRNeYS United States Patent O MULTI-BAND TUNEDANTENNA Gordon Alfred Bird, Croydon, England Application September 20,1956, Serial No. 611,018

Claims priority, application Great Britain November 23, 1955 7 Claims.Cl. 343-747 This invention is concerned with aerials and in particularto aerials which are effective at a plurality of frequencies or over aplurality of bands of frequencies simultaneously.

Radio stations are frequently required to transmit and/or receive on aplurality of frequencies or within a plurality of bands of frequencies.Generally it is necessary, in order that transmission and/or receptionshall be as effective as possible, that a separate aerial be employedfor each frequency or band of frequencies.

When a directional aerial, that is an aerial which is more effective fortransmission or reception in one direction than others, is to beemployed it is usually effective only at one frequency or over one bandof frequencies.

The plurality of aerials' which are thus necessary at such a radiostation require a considerable space in which they can be erected. Ifthe various aerials are erected in close proximity to each other thecharacteristics of any one aerial will be affected by the presence ofthe other aerials.

This latter effect causes difiiculties when the radio station is locatedin a confined area, such for example as often occurs in the case of anamateur radio station. In some cases the space available may evenprecludethe erection of more than one aerial.

It is an object of the invention to overcome or minimize the aforesaiddifiiculties.

More particularly it is an object of the invention to provide an aerialsystem which is automaticallyeffective for operation on any one of theplurality offrequencies or bands of frequencies without adjustment.

The invention provides an aerial comprising anelectric conductorresonant at a first frequency and having associated therewith a firstreactance means whereby the aerial is resonant at a second frequency anda second reactance means whereby at the first frequency the firstreactance means is rendered ineffective whereby the aerial is resonantat both the first and second frequencies;

The invention also provides an aerial which is re'so nant at each of aplurality of frequencies and comprises a conductor resonant at a firstfrequency, a first reactance means whereby the conductor is resonant ata second frequency and a second reactance means whereby the firstreactance means is rendered ineffective at the first frequency. v p

Further, the invention provides a directive aerialwhich is effective ata plurality of frequencies and which c'omprises a plurality of electricconductors at least one of which is resonant at a first frequency andhas asso= ciated therewith a first reactance means whereby it isresonant at a second frequency and a second reactance means whereby thefirst reactance means is rendered ineffective at the first frequency.

'Still further,v the invention provides a directive aerial which"is'effective at each of three frequencies and which comprises a drivenelement and two parasitic elements; each parasite element comprising aconductor resonant at one of the three frequencies and having associatedtherewith a first reactance means whereby the element is resonant atanother of said three frequencies arid a second reactance means wherebythe first reactance means is rendered ineffective at the one frequency.

In order that the invention shall be more readily understood oneparticular embodiment thereof will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a directive aerial, in accordancewith the invention,

Figure 2 is a schematic diagram of one form of inipedance transformeradapted for use with an aerial according to the invention, 1

Figure 3 is a view of the aerial shown schematically in Figure 1 showingthe mechanical arrangement thereof,

Figure 4 is a plan view of another directive aerial in accordance withthe invention,

Figure 5 is a view of a modified driven element for an aerial inaccordance with the invention, and

Figure 6 is a view, partly in section, of another aerial in accordancewith the invention.

The embodiment shown in Figure 1 is arranged as a directive aerial, thatis an aerial which is more efiec tive for transmission or reception inone direction than in others.

This embodiment is a three element beam aerial system comprising aradiating element 1, a director element 2 and a reflector element 3. Thesystem is arranged to be effective ov'er three frequency bands namelythe 14, 21 and 28 mc./S. amateur bands. Each of the elements of thesystem will be described separately.

The resonant frequency of an element can be varied by varying its lengthor by adding capacitance or inductance in series with the conductors ofthe element. When capacitance is added the resonant frequency of i theelement will be increased whilst if inductance is added the resonantfrequency will be decreased.

The first element which will be described is the reflector 3. Thiselement is of a length such that it is effective at about 21 r'nc./s.The elementis cut approximately in the centre and the two resultinghalves 4, 5 are connected by a first reactance means comprising aninductor 6 the inductance of which is such that the element is theneffective at a frequency a little below 14 mc./s.

This inductor has to be rendered ineffective before the reflectorelement 3 is again effective at 21 mc./s. To do this a second reactancemeans is arranged to present alow impedance path in shunt with theinductor 6;

The low impedance path is provided by a quarter wave stub 7 resonant atabout 21' mc./s. This stub comprises a length of low impedance twinfeeder presenting two substantially parallel conductors eachapproximately a quarter wave length long at 21 mc./s. One end of eachconductor is connected to an end ofthe inductor. The other ends of eachconductor are left unconnected. At 21 mc./ s. the stub presents alowimpedance path inshunt with the inductor thus rendering it ineffectiveat this" frequency. At 14 mc./s. the stub will behave as a smallcapacity shunting the inductor. The effect of this'is that the inductorneed possess less inductance to ma'lietlie reflector element remaineffective at 14 mc./s.

At 28' mc./s. the reflector element is such that it behaves somewhat asif it were physically three half wave lengths long, the centre half waveportion and pa'rto'f each of t-he-t-wo remaining half wave portionsbeing-represented electrically bythe inductor and thus relativelyineffective: The effect of this is to produce at 28' mcl/s'. two currentmaxima in the reflector element, one about eight feet fr'o'rri each endso that the reflector element effectively presents twig) half wavereflectors operating in phase with each ot er.

It will be seen that the reflector element is now simultaneouslyeffective at 14, 21 and 28 mc./ s. The length of this element is 23feet.

The next element which will be described is the director element 2. Thiselement is very similar to the reflector element. However in this casethe physical length of the element 2 is such that it is effective atabout 28 mc./s. The element is cut approximately in the centre and thetwo resulting halves 8, 9 connected by an inductor 10 which has aninductance such that the element is also effective on 21 mc./s. Aquarter wave stub 11 is again connected across the inductor; in thiscase the stub is resonant at about 28 mc./s. The length of the directorelement is 16 feet.

The driven element 1 which is centre fed will now be described. It iscustomary to use a driven element which .is about a half wave lengthlong at the desired frequency of operation but this is by no meansessential as the physical length'of the driven element has little effecton either the gain or the front to back ratio of the aerial system. Theonly effect of changing the physical length is to alter the feedimpedance and band width of the system.

The length of the driven element chosen in the present embodiment isabout 24 to 26 feet. This length is such that it is etfective at 14, 21and 28 mc./s. although the driven element is aperiodic at thesefrequencies. At 28 mc./s. there is a current maximum in each of the twoportions of the element which lie behind the director element. At 21mc./s. this length is such that the feed impedance is advantageouslyincreased.

The'driven element 1 is fed approximately at its centre "by means of anopen wire twin tfeeder of about 450 ohms impedance. This impedance givesthe lowest average standing wave ratio over the three frequency bands atwhich the aerial is effective. The losses in this type of feeder due tostanding waves are much lower than in the case 'of a low impedance twinwire or coaxial feeder.

The spacing between the driven element 1 and the director element 2 isfeet and the spacing between the driven element 1 and the reflectorelement 3 is 7 feet.

An aerial tuning unit is used to couple the aerial system to atransmitter or receiver. By a suitable choice of lengths of the feederthe aerial tuning unit can be arranged to act as an impedancetransformer which is effective to transform the impedance presented bythe feeder to a substantially constant, predetermined impedance at eachof the three frequencies at which the aerial is effective.

The'length of feeder suitable is found by determining a point on thefeeder which is approximately a number of quarter wavelengths from theend of the driven element to which the feeder is connected at each ofthe frequencies at which the aerial system is effective. In the presentembodiment a suitable convenient length is between 38 and 40 feet.

Figure 2 is a schematic diagram of the aerial tuning unit whichcomprises an inductor having two parts 12, 13 and capacitor 14 connectedin series, the free end of the inductor parts 12, 13 being connectedeach to one of terminals 15, 16 to each of which one wire of the feederis connected. When a suitable length of feeder is connected to theaerial tuning unit the reactance of the feeder in combination with thereactances of the inductor and capacitor in the aerial tuning unitcombines automatically to resonate the arrangement on each of the threebands of frequencies. The capacitor 14 is a trimmer capacitor providedfor tuning out any reactance which may be introduced due to incorrectchoice of feeder length and after initial adjustment need rarely beadjusted further.

A further inductor 17 is arranged in the aerial tuning unit to, provideinductive coupling to a low impedance feeder at terminals 18, 19 andwhich connects the aerial tuning unit, to the transmitter or receiver.In order to provide a relatively uniform coupling on each of the threebands the inductance of the further inductor 17 is such as v to giveadequate coupling on 21 and 28 mc./s. but insufficient coupling on 14mc./ s. To increase the coupling on 14 mc./s. to an adequate value acapacitor 20 is included in series with the further inductor, thecapacitance of the capacitor being such as to resonate the furtherinductor at about 14 mc./s.

It will be seen that the aerial coupling unit is simple and thus can bemade relatively compact and is capable of operation, in conjunction withthe aerial system, on any of the three bands without adjustment.

The mechanical construction of the aerial system will now be describedin connection with Figure 3.

A horizontal boom 21 is arranged to carry the three elements of theaerial system. This boom 21 is supported on a mast 22 or any othersuitable support. On the underside of the boom three short, relative tothe elements, inverted U-section metal channels 23, 24 and 25 arearranged substantially parallel to each other approximately at rightangles to the boom 21 and spaced along the length of said boomsubstantially in the same disposition as the three elements 1, 2 and 3.The elements are supported from beneath the U-section channels by meansof standoff insulators 26. By arranging the U-section channels in thisway they form protective covering for the inductors associated with thedirector and reflector elements.

The inductors 6 and 10 are coils wound of heavy gauge copper wire ortubing. If desired they can be further protected from the weather bycovers of a material such as polythene.

The stubs, which are of low impedance twin feeder as described above,are passed down the inside of the boom 21 where they are protected fromthe weather and also hidden from sight so that the aerial system is of atidy appearance.

The feeder, by which the driven element is connected to a transmitter orreceiver, is shown at 38.

Figure 4 shows a further embodiment of the invention which is adirective aerial effective on two frequencies or hands of frequencies.In this arrangement the driven element 27 and a parasitic element 28,which is arranged to be effective as a reflector element, are eacharranged in two halves each of which is secured at one end to one of twosubstantially parallel metal members 29, 30 which form the boom.

The driven element 27 is of a length such that it is resonant at thehigher frequency of the two at which the serial is effective. The drivenelement is resonated at the lower frequency by means of a firstreactance means which comprises an inductive reactance in the form ofpart of the two metal members 29, 30 between the ends of each half ofthe driven elements 27 and an adjustable shorting bar 31. The shortingbar 31 is adjustable along the length of the members 29, 30 thereby tovary the inductive reactance presented thereby. A second reactance meansis arranged in shunt with the first reactance means to render itineffective at the higher frequency. This second reactance meanscomprises a quarter wave stub at the higher frequency, 32, and is lengthof twin feeder.

The reflector element is similar to the driven element. In this case thefirst reactance means for the reflector element is variable by means ofthe shorting bar 33 and the second reactance means comprises a furtherquarter wave stub at the higher frequency, 34, which is also a lengthtwin feeder.

As will be seen from the drawing both the second reactance means 32 and34 are arranged inside the metal members 29 and 30.

The aerial is supported upon a mast (not shown) or a similar support bymeans of the support member 35.

The driven element is connected to a transmitter or receiver by means ofa coaxial feeder 36, the outer conductor of which is connected to theshunting bar 31 or another part of the metal member 29. The inner conductor is connected to one end of a conductor 37 through a capacitor 37athe other end of the conductor 37 being connected to one half of thedriven element 27 part way between its ends. The conductor 37 forms animpedance transformer "between the coaxial feeder and the driven element27.

It will be appreciated that the arrangement of the first reactance meansas spaced, substantially parallel conductors electrically connected toone end shown in Figure 4 is equally applicable to the arrangement shownin Figure 3 if the boom 21 is replaced by two metal members similar tothe members 29 and 30 shown in Figure 4.

In Figure there is shown an arrangement of a driven element, which canbe used with the arrangements shown in Figures 3 and 4, in which thedriven element is effective as a half-wave dipole at each of thefrequencies at which the aerial is effective.

The driven element shown in Figure 5 comprises a conductor 39 which iselfecetive as a half-wave dipole at the highest frequency at which theaerial is effective. The inductors 40 are arranged to present a highimpedance at this highest frequency thus isolating the remainder of thedriven element from the conductor 39 at the highest frequency.

To each of the inductors 40 there is connected a further conductor 41.These conductors is conjunction with the conductor 39 are effective as ahalf-wave dipole at the next highest frequency at which the aerial iseffective. At this frequency the inductors 40 present a low impedancerelative to that which they present at the highest frequency and so areineffective. It will, of course, be appreciated "that at the nexthighest frequency the inductors have some loading efiect so that thecombined lengths of the condoctors 39 and 41 will be somewhat less thana half-wave length long at the next highest frequency.

The inductors 42 act at the next highest frequency similarly to theinductors 40 at the highest frequency. The additional conductors 43together with the loading effects of inductors 42 and the remaininginductors 40 and conductors 39 and 41 make the driven element effectiveat the lowest frequency at which the aerial is effective.

The conductors 44 which are arranged parallel to the conductor 39 areconnected to the conductor by means of connections 4-5. This arrangementconstitutes an im pedance transformer between the co-axial feeder 46 andthe driven element.

Figure 6 shows a further embodiment of the invention as applied to anaerial which is effective at both band I and band III to enablereception of a television signal in each band.

The conductors 47, which are hollow tubes, are arranged as a half wavedipole at the desired frequency in band III. The inductor 48, which iswound upon a core of ferrite material, acts as a loading coil inconjunction with the conductors 47 so that they are resonant at thedesired frequency in band I. At the band III frequency the inductor 48is rendered ineffective by means of two quarter wave stubs 49 which area quarter wave at the band lII frequency. The feeder 50 is connected totapping points on the inductor 48. It will be seen that the arrangementof Figure 6 provides a two-band television aerial to which connection ismade by a single feeder and which is physically small, the length of theconductors 47 being determined by the higher frequency which is to bereceived.

It will be appreciated by those skilled in the art that the drivenelement can also be made resonant at a plurality of frequencies in thesame manner as the reflector and director elements.

Further, an impedance network can be used at the feed point of thedriven element automatically to transform the impedance of said drivenelement to a low impedance at each of said resonant frequencies of theaerial system.

I claim:

1. An antenna resonant at a first frequency and a second frequencycomprising two spaced elements, a reacltance means connecting an end ofone element to an adjacent end of the other element whereby the antennais resonant at said first frequency, and a transmission line anelectrical quarter wave length long at said second frequency having twoends, one of said line ends being free of electrical connections and theother of said line ends being connected electrically in parallel withsaid first reactance means whereby the antenna is resonant at saidsecond frequency.

2. A directive antenna eflective at a plurality of frequencies whichcomprises a driven element which is resonant at a frequency differentfrom any of said plurality of frequencies, a parasitic reflector elementand a parasitic director element, said parasitic reflector and directorelements each comprising two "spaced conductive members, a reactancemeans connecting an end of one conductive member to an adjacent end ofthe other conductive member whereby the antenna is resonant at saidfirst frequency and a transmission line an electrical quarter wavelengthlong at said second frequency and connected electrically in parallelwith said reactance means whereby the antenna is resonant at said secondfrequency.

3. A directive antenna effective at first, second and third frequencies,comprising a driven element which is resonant at a frequency differentfrom any of said first, second and third frequencies, a parasiticreflector element resonant at said first and second frequencies andincluding two spaced conductive members, a first reactance meansconnecting an end of one conductive member to an adjacent end of theother conductive member whereby said reflector element is resonant atsaid first frequency, a transmission line an electrical quarterwavelength long at said second frequency and connected electrically inparallel with said first reactance means whereby said reflector elementis resonant at said second frequency, a parasitic director elementresonant at said second and third frequencies and including two spacedconductive members, a second reactance means connecting an end of oneconductive member of the director element to an adjacent end of theother conductive member of the director element whereby said directorelement is resonant at said second frequency, and a transmission line anelectrical quarter wavelength long at said third frequency and connectedelectrically in parallel with said second reactance means whereby saiddirector element is resonant at said third frequency.

4. A directive antenna effective at first, second and third frequenciescomprising a driven element which is resonant at a frequency differentfrom any of said first, second and third frequencies, a parasiticreflector element resonant at said first and second frequencies andincluding two spaced conductive members of substantially equal length, afirst inductor connecting an end of one conductive members to anadjacent end of the other conductive member whereby the reflectorelement is resonant at said first frequency, a transmission line anelectrical quarter wavelength long at said second frequency andconnected electrically in parallel with said first inductor whereby thereflector element is resonant at said second frequency, a parasiticdirector element resonant at said second and third frequencies andincluding two spaced conductive members of substantially equal length, asecond inductor connecting an end of one conductive member of thedirector element to an adjacent end of the other conductive member ofthe director element whereby the director element is resonant at saidsecond frequency, and a transmission line an electrical quarterwavelength long at said third frequency and connected electrically inparallel with said second inductor whereby said director element isresonant at said third frequency.

5. A directive antenna effective at first, second and third frequenciescomprising a driven element, a parasitic reflector element and aparasitic director element, said driven element comprising two spacedconductive members of substantially equal length terminating in twoadjacent ends and a twin feeder having a length substantiallycorresponding to a number of electrical quarter wavelengths at each ofsaid first, second and third fre quencies and connected to said adjacentends of said spaced conductive members, said parasitic reflector elementbeing resonant at said first and second and comprising two'conductivemembers of substantially equal length terminating in two adjacent ends,a first inductor connecting said adjacent ends of the conductive membersof the parasitic reflector element whereby the reflector element isresonant atsaid first frequency, a first transmission line an electricalquarter wavelength long at said second frequency and connectedelectrically in parallel with said first conductor whereby the reflectorelement is resonant at said second frequency, said parasitic directorelement being resonant at said second and third frequencies andcomprising two spaced conductive members of substantially equal lengthterminating in two adjacent ends of the conductive members of thedirector element whereby the director element is resonant at said secondfrequency, and a second transmission line an electrical quarterwavelength long at said third frequency and connected electrically inparallel with said second inductor whereby the director element isresonant at said third frequency.

6. A directive antenna effective at first, second and third frequenciescomprising a driven element resonant at a frequency different from saidfirst, second and third frequencies, a parasitic reflector element and aparasitic director element, said driven element including two spacedconductive members of substantially equal length terminating in twoadjacent ends, an impedance transformer, a

twin wire feeder having a length substantially corresponding to a numberof electrical quarter wavelengths at each of said adjacent ends and saidimpedance transformer, said parasitic reflector element comprising twospaced conductive members of substantially equal length terminating intwo adjacent ends, a first inductor connecting said adjacent endswhereby, the reflector element is resonant at said first frequency, atransmission line an electrical quarter wavelength long at said secondfrequency and connected electrically in parallel with said firstinductor whereby the reflector element is resonant at said secondfrequency, said parasitic director element including two spacedconductive members of substantially equal length terminating in twoadjacent ends, a second inductor connecting said adjacent ends wherebythe director element is resonant at said second frequency, atransmission line an electrical quarter wavelength long at said thirdfrequency connected electrically in parallel with said second inductorwhereby the director element is resonant at said third frequency, saidimpedance transformer including a third inductor presenting twoportions, a capacitor connecting one end of one portion to one end ofthe other portion, said twin wire feeder connecting the other ends ofeach of said portions to said spaced conductive members of said drivenelement, a pair of terminals, and a fourth inductor electromagneticallycoupled to said third inductor and connecting said pair of terminals forpresenting between them a substantially constant,

predetermined impedance.

7. A directive antenna effective at first, second and third frequenciescomprising a driven element resonant at a frequency different from saidfirst, second and third frequencies, a parasitic reflector element, aparasitic director element, a supporting boom for said elements, saidsupporting boom including two substantially parallel conductive members,said driven element including two spaced conductive members ofsubstantially equal length, insulators insulatingly supporting each ofsaid conductive members of the drive members from one of saidsubstantially parallel conductive members, said conductive members ofthe driven member terminating in two adjacent ends, an impedancetransformer, a twin wire feeder having a length substantiallycorresponding to a number of electrical quarter wavelengths at each ofsaid first, second and third frequencies and connected between saidadjacent ends and said impedance transformer, said parasitic reflectorelement including two spaced conductive members of substantially equallength terminating in two adjacent ends and each carried by one of saidsubstantially parallel conductive members, a first shorting bar couplingsaid substantially parallel conductive members for forming a firstinductive reactance connecting the adjacent ends of said two conductivemembers whereby the reflector element is resonant at said firstfrequency, a transmission line an electrical quarter wavelength long atsaid second frequency and connected electrically in parallel with saidfirst inductive reactance whereby said reflector element is resonant atsaid second frequency, said parasitic director element including twospaced conductive members of substantially equal length terminating intwo adjacent ends and each supported by one of said substantiallyparallel conductive members for forming a second inductive reactanceconnecting said adjacent ends of the conductive members of the directorelement whereby the director element is resonant at said secondfrequency, a transmission line an electrical quarter wavelength long atsaid third frequency, said impedance transformer including a thirdinductive reactance presenting two portions each having two ends, acapacitor connecting one end of one of said portions to one end of theother of said portions, said twin wire feeder connecting the remainingends of said portions to the conductive members of said driven element,a pair of terminals, and a fourth inductive reactanceelectromagnetically coupled to said third inductive reactance andconnecting said pair of terminals --for presenting between them asubstantially constant, predetermined impedance.

References Cited in the file of this patent UNITED STATES PATENTS

